Polycrystalline silicon, also known as polysilicon, is generally deposited by thermal decomposition of silane (SiH4) at a low pressure in a conventional semiconductor manufacturing process. A polysilicon layer is commonly used in the formation of a memory device, such as a flash memory device. In a flash memory device, a polysilicon layer may be provided for the formation of the floating gate and/or the control gate.
One popular conventional technique of providing a polysilicon layer is low pressure chemical vapor deposition (“LPCVD”) because of its lower cost, higher production throughput, and superior film properties. Polysilicon is deposited at around 620° C. Alternatively, amorphous silicon may be deposited at a lower temperature, for example, 560° C. The amorphous silicon may then be transformed into polysilicon through annealing. However, the annealing process often produces a polysilicon layer with large grain boundaries, which may cause electron trapping at the polysilicon/gate oxide interface, and adversely affect the operations of the memory device. The cause of electron trapping is illustrated in FIG. 1.
FIG. 1 is a cross-sectional view of prior art semiconductor device. Referring to FIG. 1, a gate oxide 12 is formed on a substrate 10. A polycrystalline silicon film 14 provided through the LPCVD process is formed over gate oxide 12. As is known, polycrystalline silicon film 14 is made up of silicon crystallines separated by grain boundaries 16. One of the problems associated with the deposition of a polycrystalline silicon film is the formation of undesirable oxide valleys 18 at the interface between gate oxide 12 and polycrystalline silicon film 14. Valleys 18 may cause unintended trapping of electrons, particularly during erase operations, and causes non-uniform performance of the flash memory cell.